1. Field of the Invention
The present invention relates to piezoelectric thin-film devices and particularly relates to a piezoelectric thin-film multilayer body including a lead-free piezoelectric material.
2. Description of the Related Art
A piezoelectric device is a device that utilizes the piezoelectric effect of a piezoelectric material and has been widely used as a functional electronic part such as an actuator that generates a displacement or a vibration upon the application of a voltage to a piezoelectric material or a stress sensor that generates a voltage upon the deformation of a piezoelectric material caused by a stress applied thereto. Hitherto, a lead zirconate titanate-based perovskite-type ferroelectric (composition formula: Pb(Zr1-xTix)O3, referred to as “PZT”), which has a good piezoelectric property, has been widely used as a piezoelectric material used for an actuator or a stress sensor.
PZT is a specified hazardous substance containing lead but is exempt from the Directive of the European Parliament and of the Council on the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS directive) because, at present, there is no commercially available alternative to PZT that could be suitably used as a piezoelectric material. There has been a growing international demand for global environmental conservation, and accordingly development of a piezoelectric device including a piezoelectric material containing no lead (i.e., lead-free piezoelectric material) is strongly anticipated.
On the other hand, as a result of recent progress in the reduction in size of and enhancement of the functionality of various electronic equipment, there has been also a demand for both a reduction in the size of and enhancement of the functionality of piezoelectric devices. In an existing piezoelectric device produced by a powder sintering method, when the thickness of the piezoelectric material is 10 μm or less, it becomes impossible to ignore the effect of the crystal grain boundaries on the piezoelectric property because the thickness of the piezoelectric material is comparable to the size of the crystal grains constituting the piezoelectric material. Specifically, the positional relationship between an electrode and a crystal grain boundary and the density of the crystal grain boundaries vary among sintered piezoelectric materials, which results in significant variations among the piezoelectric properties of piezoelectric devices. In response to these issues on the reduction in size (i.e., reduction in thickness) of a piezoelectric device, there has recently been proposed a piezoelectric thin-film device produced by utilizing a thin-film deposition technique instead of a powder sintering method.
An example of a piezoelectric thin-film device including a lead-free piezoelectric material is a piezoelectric thin-film device disclosed in Japanese Unexamined Patent Application Publication No. 2007-019302 (hereinafter, referred to as “PTL 1”), which includes a lower electrode, a piezoelectric thin film, and an upper electrode stacked on or above a substrate. The piezoelectric thin film is a dielectric thin film composed of an alkaline niobium oxide-based perovskite compound represented by the following general formula (NaxKyLiz)NbO3 (0<x<1, 0<y<1, 0≦z<1, x+y+z=1). The piezoelectric thin-film device further includes, as a buffer layer interposed between the piezoelectric thin film and the lower electrode, a thin film composed of a material that has a perovskite-type crystal structure and is likely to be oriented in the direction of the (001), (100), (010), or (111) plane with a high degree of orientation. According to PTL 1, the piezoelectric thin-film device including a lead-free, lithium potassium sodium niobate thin film produces a sufficiently good piezoelectric property.
Generally, the size of crystal grains in a piezoelectric thin-film device, which is produced by utilizing a thin-film deposition technique, is very small. Therefore, the deviation of the density of the crystal grain boundaries of the piezoelectric thin film is small, which reduces the variations in piezoelectric property due to the presence or absence of a crystal grain boundary. On the other hand, since the size of the crystal grains is small and the number thereof is large, the orientation of crystal grains (i.e., degree of crystal orientation in the piezoelectric thin film) greatly affects the piezoelectric property and the reliability (e.g., variations and service life) of the device. Generally, the orientation of a thin film deposited on or above a substrate is likely to be affected by the base material.
An example of a technique for enhancing the piezoelectric property of a piezoelectric thin-film device is a piezoelectric thin-film device disclosed in Japanese Unexamined Patent Application Publication No. 2010-161330 (hereinafter, referred to as “PTL 2”), which includes a lower electrode, a piezoelectric layer, and an upper electrode stacked on or above a substrate. The piezoelectric layer includes a lead-free niobium oxide-based perovskite-type crystal as a main phase. The lower electrode has a surface roughness of 1.1 nm or less in terms of root-mean-square roughness Rms. According to PTL 2, a piezoelectric thin-film device having a good piezoelectric property can be realized by controlling the surface roughness of the lower electrode to be in a specific range.
Japanese Unexamined Patent Application Publication No. 2000-285626 (hereinafter, referred to as “PTL 3”) discloses a piezoelectric thin film including a substrate having a surface roughness (Ra) of 0.05 μm or less and a piezoelectric thin film deposited on the substrate. According to PTL 3, a high-performance piezoelectric thin-film device can be realized by controlling the smoothness of the substrate surface and thereby improving the piezoelectric property of the piezoelectric thin film.